Fix push to hub to allow individual split push
# Description of the issue

If one decides to push a split on a datasets repo, he uploads the dataset and overrides the config. However previous config splits end up being lost despite still having the dataset necessary.

The new flow is the following:
 - query the old config from the repo
 - update into a new config (add/overwrite new split for example)
 - push the new config

# Side fix

 - `repo_id` in HfFileSystem was wrongly typed.
 - I've added `indent=2` as it becomes much easier to read now.

